Light might make you a lightweight—in a good way. It’s been known that bright light in the morning can reduce appetite and body weight. But that fact did not prove that light has a direct effect on weight. Early morning exposure to light could just be a marker for a regular sleep cycle, which is also associated with a healthy body weight.

The question was thus whether light exposure was associated with weight regardless of sleep patterns.

To find out, researchers had 54 adults record their diet and sleep for a week. The subjects also wore sensors that monitored the timing and intensity of their light exposure.